1. Field of the Invention
The present invention relates to nonreciprocal circuit devices, and more particularly, to a structure of a micro-wave nonreciprocal circuit device for use as an isolator or a circulator.
2. Description of the Related Art
In general, in an isolator or a circulator, a signal is passed only in a transfer direction and opposite-direction signal transfer is blocked. They are, for example, employed in transmitting and receiving circuit sections of mobile communication equipment such as a portable telephone or a car telephone. It has been increasingly demanded that an isolator and a circulator used for this purpose have advanced performance.
FIG. 8 is an exploded perspective view of a central electrode assembly of a three-port-type isolator which has been used. A central electrode assembly 101 has a structure in which central conductors 103 to 105 are formed on the upper surfaces of insulating layers 102 which in whole or in part are made from ferrite. The insulating layers 102 are laminated such that the central conductors 103 to 105 overlap with each other at an angle of about 120 degrees, and the insulating layers are sandwiched by a pair of ferrites 106. The central conductors 103 to 105 are each formed of one strip conductor as shown in the figure and serve as ports.
To increase the Q value of a conductor, it is demanded that the thickness of each of the strip conductors 103 to 105 be more than about three times the skin depth. In the structure described above, however, if strip conductors are formed such that the thickness is more than three times the skin depth, large gaps are formed between the various layers 102 and 106 when the layers are laminated and they are likely to break.
To solve the foregoing problem, a method has been proposed in which a central conductor serving as a port is formed of a plurality of strip conductors on respective insulating layers. The strip conductors and insulating layers are sequentially laminated for each central conductor as shown in FIG. 9. With this method, even if the thickness of one strip conductor is not more than three times the skin depth, because a plurality of strip conductors are laminated, the same Q value is obtained as in a case when the conductor is formed to have the required thickness. Therefore, the thickness of each strip conductor can be small, while the required Q value is obtained, and the risk of breakage of an insulating layer is eliminated.
The above conventional nonreciprocal circuit device, however, has a problem: The coupling between central conductors is weak. FIG. 10 is a view showing a typical coupling condition between strip conductors in a nonreciprocal circuit device. In FIG. 10, the solid lines with arrows indicate a magnetic field formed by strip conductors 113a and 113b. The magnetic field is stronger at points closer to the strip conductors 113a and 113b and is weaker at points farther from the strip conductors. When a central conductor 113 and a central conductor 114 are magnetically coupled in this structure, since the magnetic field formed by the strip conductors 113a and 113b is weak near the strip conductors constituting the central conductor 114, especially near the strip conductor 114b, the magnetic coupling between the central conductors 113 and 114 becomes weak. Capacitive coupling C is achieved only between the strip conductor 113b and the strip conductor 114a, between the two central conductors 113 and 114.
As described above, electromagnetic coupling is generally weak between the two central conductors and the insertion loss of the central conductors becomes large.